Benton,

At this time you cannot use GPS coordinates to assist in the mapping. We do have plans to overhaul the interface for mapping properties, and plan to include a way to click on a single point that you have plotted, and expose the latitude/longitude coordinates. From there you would be able to enter new coordinates to refine the position of the point as long as it was within a certain proximity to the previous coordinate. The details are not all worked out yet and unfortunately I don't have a timeline for this feature yet, but be on the lookout for it in the future.